1. Field of the Invention
This invention relates to a compression molding method capable of regulating compressive force exerted on molding material filled in a mold cavity on the basis of the internal die pressure of the molding material in the mold cavity.
2. Description of the Prior Art
Molding plastic material in the molten state remarkably shrinks as it is solidified. In injection molding, in order to compensate for the shrinkage of the molding material, the capacity of a mold cavity is designed in anticipation of occurrence of the shrinkage and, in addition, the molding material in the mold cavity is forcibly compressed by a dwelling force. If the shrinkage is compensated for by utilization of the dwelling force, however, it is impossible to avoid generation of a strain in the vicinity of the gate unless a direct gate is adopted. Even if the resin pressure in the vicinity of the gate is sufficiently large, the viscosity of the molten resin is increased as the time passes and therefore, a drop in resin pressure becomes conspicuous with the movement of the molten resin in the mold cavity, thereby preventing the molten resin from maintaining a sufficiently large pressure until the mold cavity is completely filled with the resin. As a result, it is impossible to produce a uniform internal die pressure in the mold cavity. For example, a molding die having a pin gate at which molten resin is solidified quickly suffers a disadvantage that dwelling force cannot be applied to the molding material after the gate is sealed off. To eliminate this drawback, a compression molding method in which solidified resin in a mold cavity is further compressed may be applied to an injection molding machine having high productivity.
At present, compression molding machines, particularly injection compression molding machines, can be classified into two types, one in which the compressive force is produced by a molding clamping mechanism and the other in which the compression force is produced by an extruder other than the mold clamping mechanism. Of those adopting the mold clamping mechanism, there has so far been known the Rolinx process developed by Rolinx Company of England. The Rolinx process effects a mold clamping under high pressure after the mold is closed at low pressure and slightly opened by the force involved in the injection process. However, in this Rolinx process, the degree of the mold opening depends on the injection force, but the molding opening cannot be precisely controlled.
A sandwich press method which adopts a toggle type molding clamping system has been proposed by Engel Company. In the sandwich press method, the injection process is carried out while the toggle system is incompletely stretched and thereafter, compression is effected for molding material filled in the mold cavity while stretching the toggle system.
Recently, there has been developed a molding system in which the movable molding die is restricted by means of a mechanical stop to limit the movement thereof to a fixed extent.
However, the degree of shrinkage cannot be precisely regulated according to the filling condition. Consequently, sufficient compressive force cannot be used in case of a short injection short or there is a possibility that the molding die will be damaged in case of over-packing.
Further, a method using an extruder, for example a micromolding method, has been known. In this method, an oil pressure cylinder is used in addition to the mold clamping mechanism and the resin filled in the mold cavity is compressed by the oil pressure cylinder. However, the micromolding method requires an oil pressure cylinder of a large size to the extent of ignoring injection pressure and moreover, the molding die is restricted as to its construction. Besides, this known method has suffered a disadvantage that the degree of mold opening depends on injection pressure and cannot be precisely controlled.
What should be noteworthy is the fact that the filling conditions cannot constantly be maintained and varies with every injection shot. The shrinkage condition of the molding materials is affected by the filling condition and depends on the molding conditions such as the temperature of the molding die and the oil pressure in the compression system. Furthermore, these molding systems are influenced by various disturbances with the result that the molding conditions described above cannot be constantly maintained and the shrinkage of the molding material filled in the mold cavity can not be kept at a constant state. If a fixed compressive pressure is applied to the molten resin in the mold cavity at all times, the reproducibility is reduced unless the shrinkage state of the molding material is taken into account.
All the injection molding methods hereinbefore described utilize a fixed compressive pressure applied to the molding material filled in the mold cavity, and therefore they cannot regulate the compressive force in proportion to the shrinkage of the molding material.
Now, what does matter at this point is the view point from which the filling condition of molding material and the shrinkage state thereof are viewed. The inventor of this invention has taken note of the resin pressure which is considered to be a function of the plastic and has succeeded in deriving the internal die pressure from the resin pressure continuously detected in the resin passageway of a molding die as proposed in Japanese Patent Application Public Disclisure Sho. 52(1977)-14658. The internal die pressure is detected by a sensor disposed in the resin passageway of the molding die and expressed as a waveform representing the condition under which the molding material is filled into the mold cavity and shrunk throughout the molding shot from the filling process to the dwelling process.
Therefore, the variation of the shrinkage state of the molding material can be viewed as a variation of the internal die pressure of the molding material filled in the mold cavity.